In communications networks, it may be challenging to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
One component of wireless communications networks where it may be challenging to obtain good performance and capacity is the antennas of network nodes configured for wireless communications; either to/from another network node, and/or to/from a wireless user terminal.
For example, multi-antenna transmission techniques are used in several wireless communication standards, e.g. the Long Term Evolution (LTE) telecommunications standard of the 3rd Generation Partnership Project (3GPP), in order to increase system capacity and coverage. A particular transmission mode (TM) is codebook-based precoding in which the radio base station (such as an evolved Node B, or eNB) of the network transmits one or several beam formed data streams to the wireless end-user terminals (denoted user equipment, or UE). The beam forming weights are selected from a standardized codebook based on recommendations transmitted from the UE. In order for the UE to be able to recommend beam forming weights the radio base station first transmits pre-determined reference signals which are used by the UE to estimate the complex channel matrix between the radio base station and UE. This estimate may then be used to determine which weights in the codebook that for the UE will result in the best performance for the current channel state. Since there is only a finite number of eligible beam forming weights (as dictated by the codebook), only an index needs to be transmitted back from the UE to the radio base station. This index is referred to as a precoding matrix indicator (PMI). The radio base station may then select to transmit user data with the precoding matrix recommended by the UE, or with some other precoding matrix. For example, in transmission mode 4 (TM4) the radio base station may use another precoding matrix in the codebook, while in transmission mode 9 (TM9) there is no restriction on what precoding matrix for the radio base station to use. In the latter case, the codebook is only used to feedback quantized channel state information (CSI), for example acquired from channel state information reference signals (CSI-RS), whilst the demodulation of user data relies on precoded user-specific reference signals. For this reason, TM9 is sometimes referred to as non-codebook-based precoding.
Further, transmission mode 9 (TM9), defined in LTE release 10, is designed for up to eight antennas. The structure of the LTE release 10 codebook is specifically suited for a four column dual polarized uniform linear array antenna. With such types of antennas the beam forming gain, for example expressed in terms of antenna gain, is typically the same as the number of columns, i.e., a factor four. The beam forming gain is hence limited to be in the order of the number of CSI-RS ports; in case of dual polarized antennas typically half this number.
Hence, there is a need for improved beam forming.